The last few years have witnessed a surge in mm-wave integrated circuits mainly because of the increased cut-off frequency of complementary metal oxide semiconductor (CMOS) transistors. These integrated systems target applications such as wide-band communication, remote sensing and medical imaging. Moreover, recent works have shown the possibility of implementing CMOS circuits at sub-mm-wave and terahertz (THz) frequencies, which are defined from 300 GHz to 3 THz. This is motivated by the higher communication bandwidth and better imaging resolution in addition to new applications such as integrated terahertz spectroscopy. Despite all recent advances in this frequency range, a high power tunable signal source has remained a challenging yet essential circuit block needed for the realization of a complete terahertz system.
In inductor-capacitor (LC)-resonator-based VCOs, varactors are placed in the resonator in order to tune the oscillating frequency. This tuning method works well at radio frequencies and achieves moderate tunability at lower mm-wave frequencies (<100 GHz). However, there are at least two major challenges in using varactors for frequency tuning above 100 GHz. First, at these frequencies the varactor quality (Q) factor is low. This lowers the achievable output power and degrades the phase noise performance. Secondly, as the operation frequency increases, parasitic capacitances dominate oscillator tank circuits that employ varactors, thus limiting the tuning capability of varactors. These challenges impose an important trade-off in the design of high frequency oscillators. Typically, CMOS oscillators designed for operation above 100 GHz with high output power do not use varactors. As a result, the frequency of such CMOS oscillators cannot be dynamically tuned. Conversely, tunable oscillators designed for operation above 100 GHz provide very low output powers (<1 μW) due to the use of tuning varactors. As a result of these challenges, significant power generation combined with dynamic tuning for generating signals having frequencies above 150 GHz is dominated by the use of frequency multiplier type circuits. Frequency multiplication requires a high-power external source, which is not desirable in a fully integrated tunable signal source that operates at terahertz frequencies.
To address this challenge, related art tunable signal sources for lower frequencies have focused on tuning the oscillation frequency without using varactors. Magnetically tuned and transconductance tuned VCOs are two examples where, instead of capacitive tuning, an effective inductor of an oscillator tank circuit is tuned. Both of these tuning techniques still require additional active devices inside the oscillator tank circuit. A recent work has used an interpolative-phase-tuning technique in an LC ring oscillator at the mm-wave frequency range. All of these techniques have been used to generate output powers well below the cut-off frequency of the transistors. In order to realize a high power VCO at the sub-mm-wave and terahertz band, three requirements need to be satisfied. What is needed is a tunable signal source that generates high harmonic power above the fmax of the devices making up the tunable signal source. Another requirement is that the generated power be efficiently delivered to an output load. A further requirement is a frequency tuning mechanism that will not adversely affect the generation of high harmonic power or adversely affect the efficient delivery of the high harmonic power to the output load.